The present invention relates to an improved strainer for filtering molten metal wherein the strainer comprises an inoculant pellet in operational association therewith. More specifically, the present invention relates to an improved strainer-inoculant combination providing improved straining and improved inoculation of molten iron passing there through.
Filtration of molten metal is a long practised art. Innoculation of the molten metal, particularly iron, is also a widely practiced art. It has been a long-standing desire to combine the filtration, or straining, of molten iron with inoculation.
Various treatment units have been presented in the art. Many of these utilize some form of flow restriction or redirection in an attempt to maximize contact between the molten metal and the inoculant. U.S. Pat. No. 4,511,401, for example, comprises a unit which is integral to the ladle. Molten metal is inoculated upon introduction into the ladle. While this technique is convenient, the inoculant tends to fade with time and the inoculation is therefore inefficient in such a device.
Converter chambers are described in the prior art wherein a separate inoculation chamber is employed. These typically include some form of flow restrictor, such as perforated walls or overflowing dams. Representative teachings of this type of device are provided in, U.S. Pat. Nos. 4,666,133; 4,509,979; 4,464,198; 4,391,636; 4,330,024 and 4,238,231.
These devices are deficient for several reasons. One reason is the problem associated with solid inoculant breaching the flow restrictor and being incorporated as an impurity in the, flowing molten metal. This typically occurs when the inoculant particle becomes sufficiently small to traverse the flow restrictor. These particles will then either continue to dissolve, be removed by subsequent straining or become an impurity in the solidified metal. Potential incorporation of solid inoculant in the flowing molten metal makes it difficult to determine the effectiveness of the inoculation. It is also difficult to insure that all of the metal is in sufficient contact with the inoculant to provide adequate inoculation.
Continued addition of an inoculant powder, as described in U.S. Pat. No. 4,517,019, overcomes some of the problems associated with flow restrictors. These devices are expensive to operate and the powder can agglomerate creating flow problems for the solid. The flow problems result in inconsistent powder delivery and inconsistent inoculation.
Formation of the inoculant into a filter is described in U.S. Pat. No. 5,690,161. The molten metal is caused to flow through the inoculant. As would be realized, the inoculants dissolve. As the inoculant dissolves the structure is weakened thereby leading to the possibility of breakage.
Combining an inoculant, typically in ingot form, with a filter or strainer is described in U.S. Pat. No. 3,881,937 and DE 4,318,309 A1. These represent improvements over the prior art yet they are still deficient for a number of reasons.
Incorporating an inoculant pellet into a cavity, as described in DE 4,318,309 A1 has several deficiencies. The inoculant tablet is not flush with the inlet face, or surface, of the strainer. The protruded pellet is typically a compressed powder which is highly susceptible to breakage. This creates a packaging problem requiring complex internal packing materials to protect the protruded pellet. The packaging problem is made worse by the lack of bonding between the strainer and pellet. Furthermore, the side peripheral surface of the inoculant tablet within the strainer body is not sufficiently exposed to the molten metal stream. This creates inefficient treatment. If the inoculant pellet is made flush the packaging problems decrease but the deficiencies with regard to dissolution are exasperated. Filters incorporating inoculant in a cavity have inferior inclusion removal.
Securing a pellet to the face of a filter with a protruding element, as described in U.S. Pat. No. 3,881,937 reduces the problems associated with insufficient surface area yet other problems are created. The packaging problems remain and are exasperated. Problems associated with inclusion removal efficiency remain. There is a further problem associated with the weakness of the protrusion. This weakness creates a continual packaging problem resulting in expensive packing or extensive losses during shipping.
The lack of a suitable system and method for treating molten metal has been a long felt need in the art. This need is met by the present invention.